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than 90 million[2] and continues to grow at the astounding rate of 200,000 new users per week. Numbers like that tend to be somewhat difficult to understand—if all those people impossibly stood on each other’s shoulders, they’d reach 93,000 miles into the sky! But consider that San Francisco has a population just shy of 800,000 people, and now imagine each and every one of them sitting down in front of their computers and diligently joining Facebook in the same month. The current growth rate runs at about 3% a week, and if you consider that the world’s population is only growing at a rate of about 1.14% a year, it becomes mathematically possible to calculate the Facebook Singularity: the point at which every human on the planet has been signed up and is filling your inbox with free gifts, pokes, and friend requests. Although it’s difficult to figure out how much Platform has to do with that growth, it’s notable that there have been over a billion installs of almost 24,000 apps, and that some of those apps (such as Slide’s Top Friends and FunWall and RockYou’s Super Wall) are seeing well over a million daily users.

What Exactly Is Facebook Platform?

Chances are, if you’re reading this book, you’re comfortable with the concept of an operating system (OS), be it Windows, Mac OS, or Linux. In a lot of ways, you can think of Facebook Platform as an OS for social networking. Platform provides many of the important and underlying technologies that enable the social graph, a term Facebook uses to describe a social network.

Figure 1-2. The social graph

The social graph is a representation of all the connections that make up a social network. Every member of the network has his own social graph, which represents that user’s unique set of connections to other members of the same network. The example shown in Figure 1-2 depicts the social graph of the person in the center, whom we’ll call Mark. The people in the network who are directly connected to Mark are shown slightly smaller than him, the people he’s indirectly connected to are shown even smaller, the next level even smaller, etc, etc. The social graph isn’t unique to Facebook (although it’s certainly one of the biggest on the Web); it is actually a common property of any network in which things are joined to other things. It’s a useful visualization tool to show the structure of the interlinked nodes (the topology of the network), and it can also be used to calculate the value of any one node (usually based on the number of links it has to other nodes). Although it may not seem so at first, network value is a fascinating topic, especially when it’s worth an estimated $15 billion.[3] You can figure out the value of any network by applying Metcalfe’s Law: the value of a telecommunications network is proportional to the square of the number of users of the system (n2). Robert Metcalfe, who coinvented Ethernet along with David Boggs and cofounded 3Com, first formulated his law to explain the network effects of things like joining computers or fax machines together, but it’s just as useful for explaining social networks and the Web. The math behind the law is actually pretty simple and is easily illustrated by looking at the social graph just shown. If Mark is the first person to join Facebook, the value of the network is one (12 = 1). When Mark’s friend Sarah joins, the network has become a lot more valuable because now Mark can stop poking himself and can now poke someone else. It’s so much more valuable, in fact, that the value doesn’t just increase by one but actually becomes four (22 = 4), meaning that the network is now four times more valuable to Mark and Sarah than it was to either one alone. Eventually, after every member of Mark’s very extended circle of family and friends has joined, the network has 54 million users, the value of the network is too high to calculate on most calculators (54,000,0002 = 2,916,000,000,000,000), and suddenly Microsoft is offering Mark $240 million for 1.6% of his company.

Before we get too far off topic, there’s a reason why the